Uncoupled U/Pb and <scp>REE</scp> response in zircon during the transformation of eclogite to mafic and intermediate granulite (Blanský les, Bohemian Massif)

The Fuping Complex is one of the important basement terranes within the central segment of the Trans‐North China Orogen (TNCO) where mafic granulites are exposed as boudins within tonalite–trondhjemite–granodiorite (TTG) gneisses. Garnet in these granulites shows compositional zoning with homogeneous cores formed in the peak metamorphic stage, surrounded by thin rims with an increase in almandine and decrease in grossular contents suggesting retrograde decompression and cooling. Petrological and phase equilibria studies including pseudosection calculation using thermocalc define a clockwise P–T path. The peak mineral assemblages comprise garnet+clinopyroxene+amphibole+quartz+plagioclase+K‐feldspar+ilmenite±orthopyroxene±magnetite, with metamorphic P–T conditions estimated at 8.2–9.2 kbar, 870–882 °C (15FP‐02), 9.6–11.3 kbar, 855–870 °C (15FP‐03) and 9.7–10.5 kbar, 880–900 °C (15FP‐06) respectively. The pseudosections for the subsequent retrograde stages based on relatively higher H2O contents from P/T–M(H2O) diagrams define the retrograde P–T conditions of <6.1 kbar, <795 °C (15FP‐02), 5.6–5.8 kbar, <795 °C (15FP‐03), and <9 kbar, <865 °C (15FP‐06) respectively. Data from LA‐ICP‐MS zircon U–Pb dating show that the mafic dyke protoliths of the granulite were emplaced at c. 2327 Ma. The metamorphic zircon shows two groups of ages at 1.96–1.90 Ga (peak at 1.93–1.92 Ga) and 1.89–1.80 Ga (peak at 1.86–1.83 Ga), consistent with the two metamorphic events widely reported from different segments of the TNCO. The 1.93–1.92 Ga ages are considered to date the peak granulite facies metamorphism, whereas the 1.86–1.83 Ga ages are correlated with the retrograde event. Thus, the collisional assembly of the major crustal blocks in the North China Craton (NCC) might have occurred during 1.93–1.90 Ga, marking the final cratonization of the NCC.

Section: Discussionmentioning

confidence: 97%

Petrology, phase equilibria modelling and zircon U–Pb geochronology of Paleoproterozoic mafic granulites from the Fuping Complex, North China Craton

Tang

Santosh

Tsunogae

et al. 2017

“…(). These methods have been used to great effect in a number of studies where the range of zircon and garnet chemistry and morphologies are relatively simple (Clark et al., , ; Gauthiez‐Putallaz, Rubatto, & Hermann, ; Hermann & Rubatto, ; Kotková & Harley, ; Kotková, Whitehouse, Schaltegger, & D'Abzac, ; Kylander‐Clark & Hacker, ; MacDonald et al., ; Štípská et al., ). The utility of these techniques in visualizing equilibrium relationships in terranes that have experienced multiple episodes or protracted durations of metamorphism becomes increasingly complex as zircon undergoes varying degrees of recrystallization, re‐equilibration and neocrystallization.…”

Section: Discussionmentioning

confidence: 99%

“…The relationship has been calibrated from natural mineral pairs (Buick, Hermann, Williams, Gibson, & Rubatto, ; Hermann & Rubatto, ; Hokada & Harley, ; Whitehouse & Platt, ) and experiments (Rubatto & Hermann, ; Taylor, Harley, et al., ). A number of studies have demonstrated that the zircon–garnet relationship can withstand long durations of metamorphism at high temperature (Clark, Collins, Santosh, Taylor, & Wade, ; Harley & Nandakumar, ; Hermann & Rubatto, ; Kelly & Harley, ; Štípská, Powell, Hacker, Holder, & Kylander‐Clark, ; Taylor, Clark, Johnson, Santosh, & Collins, ), further establishing the utility of zircon–garnet relationships in interrogating the evolution of metamorphic terranes. The recently established ability to rapidly acquire in situ isotopic and trace elemental data sets through techniques such as laser ablation split stream petrochronology means that large coupled geochronological–geochemical data sets can be applied to understanding high‐ T events (Horton et al., ; Kylander‐Clark, Hacker, & Cottle, ; Shaffer, Hacker, Ratschbacher, & Kylander‐Clark, ).…”

Section: Introductionmentioning

confidence: 97%

Prolonged (>100 Ma) ultrahigh temperature metamorphism in the Napier Complex, East Antarctica: A petrochronological investigation of Earth's hottest crust

Clark

Taylor

Kylander‐Clark

et al. 2018

The Napier Complex in East Antarctica preserves a record of ultrahigh temperature (UHT) metamorphism during the late Archean to early Palaeoproterozoic. While there is little argument that the UHT metamorphic event began at c. 2,580 Ma, the duration over which the rocks resided at UHT has been the subject of intense debate, with estimates for the end of metamorphism ranging from 2,545 to 2,440 Ma—a discrepancy of some 105 Ma. To resolve the time‐scale of UHT metamorphism, a zircon and garnet petrochronological (U–Pb, REE and Ti) data set from a suite of rocks from the Tula Mountains region of the Napier Complex was analysed. Individual concordant populations define zircon U–Pb ages for (a) reset zircon cores of 2,502–2,439 Ma; (b) zircon rims of 2,491–2,454 Ma; and (c) neocrystallized sector‐zoned zircon from 2,492 to 2,443 Ma. Ti‐in‐zircon thermometry places a minimum estimate of 830°C for zircon crystallization, with the majority of concordant populations yielding temperatures >900°C. Zircon–garnet partitioning (DYb vs. DYb/Gd) arrays reveal that the bulk of metamorphic zircon defines an equilibrium relationship with the garnet that forms part of the peak assemblage. Combined with existing geochronological constraints, the new petrochronological data demonstrate that the Napier Complex remained at UHT from c. 2,585 Ma until at least 2,450 Ma, a residence time of 135 Ma. In the absence of evidence for contemporaneous emplacement of large volumes of igneous rocks, a number of factors likely combined to drive and maintain these extreme temperatures. We propose that the P–T conditions experienced by the Napier Complex were achieved through a combination of orogenic plateau formation, preconditioning of the crust by a high‐T magmatic and UHT metamorphic event at c. 2,850 Ma, inefficient removal of heat‐producing elements during partial melting and slow exhumation. This style of long duration, regional, extreme metamorphism is becoming more commonly identified in the rock record as larger and more robust data sets are collected (e.g. the Eastern Ghats of India and the Gondwanan East African Orogen) and is commonly associated with the amalgamation phases of supercontinents/cratons.

“…Either the zircon grains were not in equilibrium with the other phases in the rock (possibly due to their position in the now‐former osumilite) or they did not grow/recrystallize at UHT. A similar lack of compositional equilibrium during resetting of zircon U–Pb dates has been documented in the granulites of Blanský Les in the Bohemian Massif (Štípská, Powell, Hacker, Holder, & Kylander‐Clark, ). All monazite from this sample is isotopically homogeneous: 561 ± 6 Ma (Figure h).…”

Section: U–pb Laser‐ablation‐split‐stream Icp‐ms Monazite and Zircon mentioning

confidence: 99%

Ultrahigh‐temperature osumilite gneisses in southern Madagascar record combined heat advection and high rates of radiogenic heat production in a long‐lived high‐T orogen

Holder

Hacker

Horton

et al. 2018

We report the discovery of osumilite in ultrahigh‐temperature (UHT) metapelites of the Anosyen domain, southern Madagascar. The gneisses equilibrated at ~930°C/0.6 GPa. Monazite and zircon U–Pb dates record 80 Ma of metamorphism. Monazite compositional trends reflect the transition from prograde to retrograde metamorphism at 550 Ma. Eu anomalies in monazite reflect changes in fO2 relative to quartz–fayalite–magnetite related to the growth and breakdown of spinel. The ratio Gd/Yb in monazite records the growth and breakdown of garnet. High rates of radiogenic heat production were the primary control on metamorphic grade at the regional scale. The short duration of prograde metamorphism in the osumilite gneisses (<29 ± 8 Ma) suggests that a thin mantle lithosphere (<80 km) or advective heating may have also been important in the formation of this high‐T, low‐P terrane.